Sub-millimeter Wave Imaging System Noise Reduction
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Solution Overview
Problem
Sub-millimeter wave imaging systems face challenges in detecting low-level signals due to high noise levels, particularly from DC bias voltage, which limits the detectability of low-power signals, and existing techniques like Chopped Detection and Phase Sensitive Detection are not fully utilized without complex semiconductor implementations.
Innovation Solution
A system that includes an antenna, a modulator responsive to a reference frequency for pulse modulating RF energy, a detector for homodyning the signal, and high pass or band pass filters to remove DC bias and low frequency noise, enabling Phase Sensitive Detection with a Low Noise Amplifier and bias modulation to reduce noise figure and enhance sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If DC bias voltage is used to establish operating point for detector, then detector sensitivity and NEP are optimized for higher RF energy levels, but a voltage floor is created that limits detectability of low-level signals
Solution Approach 1:
The patent applies periodic chopping action to modulate the DC bias current at a specific frequency. This transforms the stationary DC voltage floor into a time-varying signal that can be separated from low-level RF signals through frequency-selective filtering and phase-sensitive detection, allowing both optimized detector operation and enhanced low-level signal detectability
Solution Approach 2:
The patent introduces chopping frequency modulation as an intermediary mechanism between the DC bias current and the detector output. By modulating the bias current at a known frequency and using phase-sensitive detection referenced to this frequency, the system creates a separation pathway that allows the detector to operate at optimal bias point while rejecting the bias-induced voltage floor through frequency discrimination
2Measurement precision
If Chopped Detection is implemented using spinning perforated wheel, then DC level transformation enables high pass filtering to block DC bias voltage, but mechanical complexity increases
Solution Approach 1:
The patent replaces the mechanical spinning perforated wheel with an electronic switching mechanism that performs the same chopping function. The electronic switch modulates the DC bias current at a predetermined frequency without mechanical moving parts, eliminating mechanical complexity while maintaining the frequency modulation needed for high pass filtering and DC rejection
Solution Approach 2:
The patent changes the implementation parameter of the chopping mechanism from mechanical rotation to electronic switching. This parameter change maintains the essential function of periodic modulation while eliminating the mechanical complexity of spinning wheels, allowing for more reliable and integrable detector systems
3Measurement precision
If larger antennas are used for microwave radiation to achieve resolution, then image resolution improves, but system size increases significantly
Solution Approach 1:
The patent changes the operating frequency parameter from microwave to sub-millimeter wave range. This frequency increase allows for smaller antenna dimensions while maintaining or improving resolution, since the required antenna size is proportional to the wavelength. The sub-millimeter wave regime enables compact system design without sacrificing imaging capability
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system achieves improved sensitivity and noise reduction by transforming DC levels into pulse modulated signals, allowing for effective high pass filtering and averaging noise to zero, thereby enhancing the detectability of low-level signals and achieving better noise performance.
Implementation Method 1
a modulator, responsive to a reference frequency signal, for pulse modulating the received radio frequency energy at the reference frequency
Implementation Method 2
a detector for homodyning the pulse modulated signal to convert such pulse modulated signal to a detector output signal having a low frequency component representative of the amplitude of the received radio frequency energy
Implementation Method 3
a high pass or pass band filter at the reference frequency fed for the detector output signal for passing the high frequency components and for removing the low frequency component
Data Source
AI summary
A system for detecting the amplitude of radio frequency energy includes: an antenna for receiving the radio frequency energy; a modulator, responsive to a reference frequency signal, for pulse modulating the received radio frequency energy at the reference frequency; a detector for converting such pulse modulated signal to a detector output signal having a low frequency component representative of the amplitude of the received radio frequency energy, in summation with DC bias current, and a high frequency component at the reference signal; and a high pass or band pass filter fed for the detector output signal for passing the high frequency components and for removing the low frequency component. A phase detector, with or without a subsequent IF amplifier, is fed by the reference frequency and the high frequency components for producing an output representative of the high frequency components. A low noise amplifier is fed by the antenna and has a bias fed by the reference frequency signal for modulating the received radio frequency energy at the reference frequency.